Why are clocks normally ideal before floorplanning?

Short Answer

Before a clock tree is actually built and routed, there's simply nothing to measure — no buffers, no real wire delay, no real skew between branches, because CTS hasn't happened yet. Treating the clock as "ideal" (zero or user-specified latency, no built-in skew) isn't a shortcut or a cop-out — it's an honest reflection of what's actually known at that stage of the flow.

Technical Explanation

• Any early value you do assign — via set_clock_latency or set_clock_uncertainty — is a modeling assumption standing in for the clock tree that will eventually exist, and it should be clearly labeled and periodically reviewed as such, not treated as measured data.
• This matters because downstream engineers reading a pre-CTS timing report need to know they're looking at assumptions, not silicon-accurate numbers — conflating the two leads to false confidence in early closure.
• Once CTS actually runs (clock_opt/synthesize_clock_trees), the clock becomes "propagated" and the tool switches from the assumed ideal values to the real insertion delay and skew computed from the built tree.
• Practically: keep the assumed pre-CTS latency/uncertainty values conservative and grounded in prior tapeouts or early estimates of tree size, since an unrealistic ideal-clock assumption can mask real setup/hold problems that only show up after CTS — by which point they're much more expensive to fix.

What To Check

• Warning sign: A copied post-CTS propagated-clock setting creates fictional network behaviour.
• Inspect: Compare one named object across the related reports; the same object should tell a consistent story.
• Correct: Remove unsupported propagation and use the approved pre-CTS source latency, uncertainty, and transition model.

Healthy, Suspicious & Hard-stop Results

• Expected: There is no implemented clock tree, so built-tree skew and route latency do not exist; only justified early assumptions belong in the model.
• Stop before floorplanning when required logic or timing coverage is missing or unexplained.

What The Interviewer Is Testing

Be ready to explain why this matters before floorplanning.

Follow-up Question & Model Response

Model response: “I would save the report, inspect one affected object in the correct block and scenario, and make or request this correction. ”